@tinpanalley
1) Do what 윤희지 said. Just fully disable Internet connection by removing the Ethernet cable and not setting up WiFi. This will prevent download of any newer firmware.
2) You can perform a backup for saves using the official method available in the OS and back them up to a USB drive/portable HDD ->
https://manuals.playstation.net/document/en/ps4/settings/data_usb.html
Backing up games is a different thing. You can't back them up as easy as the saves. Basically the best method would be to connect to the network, load the FTP payload and connect with a FTP client to the FTP server on your PS4. Then you have to download the installed PKG files of the games to back them up.
Imho. If you have the installation FPKGs backing up the installed games is not really much of use. You can simply install the games again after changing HDD and import your saves later for the games. If you don't have the installation FPKGs this would be your only method to peform a backup of the installed games, unfortunately.
3) No, you can't. It won't work. Everytime you change HDDs it will ask you to reinstall FW. And once you install a newer FW than 5.05 (i.e. on your non jailbreak HDD) you can never go back to a lower FW version. Good bye jailbroken PS4 in that case. The security chip SAMU of the PS4 stores which version was installed on the PS4 and prevents downgrades.
Edit: Yes, that guide is still valid and up to date.
About your question on backing up games. See my reply to your 2nd question from your previous post. You can download a copy of the installed game via FTP payload. And saves can be backed up through the PS4 OS itself.